High pressure production of perovskites and resulting products

ABSTRACT

A perovskite feedstock (powder or preform) is placed in a high-pressure cell of a high pressure/high temperature (HP/HT) apparatus and subjected to pressures in excess of about 2 Kbar and temperatures above about 800° C. for a time adequate to increase the density of the preform.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] None

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to the production ofperovskite products, such as typified by ruthenium-based oxidecompounds, and more particularly to their densification and sinteringunder high-pressure conditions.

[0004] Simple ruthenium based oxides, for example, ARuO₃, where A is Sr,Ba, or Ca, have interesting magnetic and electrical properties. Theseoxides also have the potential for device applications. SrRuO₃, forexample, is a ferromagnet with Tc˜160 K, whereas CaRuO₃ exhibits noferromagnetic order. These ruthenium-based oxides also are electricallyconductive.

[0005] SrRuO₃, for example, has commercial application as a high densitysputtering target for a variety of emerging electronic applicationsincluding, for example, production of thin films as electrodes in thenext generation of high capacity stack storage capacitors and productionof dynamic random access memory (DRAM) and ferroelectric random accessmemory (FeRAM) for the semiconductor industry. The theoretical densityof SrRuO₃ is 6.489 g/cc. SrRuO3 targets with high density (>90% oftheoretical density) are essential for production of thin films withuniform thickness by sputtering. SrRuO₃, however, is difficult todensify by conventional methods, such as cold pressing and sintering, orhot pressing. Sintering temperatures also are limited to less than 1400°C. due to dissociation of the compound at low pressures andcontamination because of reaction with containment material. SrRuO₃compacts made by conventional densification methods are reported to havedensities of less than 50% of the theoretical density.

[0006] Thus, there exists a need in the art to provide dense compacts ofcubic perovskite products, as typified by ruthenium-based oxides.

BRIEF SUMMARY OF THE INVENTION

[0007] A perovskite feedstock (e.g., a preform or powder) is placed in ahigh-pressure cell of a high pressure/high temperature (HP/HT) apparatusand subjected to pressures in excess of about 2 Kbar and temperaturesabove about 800° C. for a time adequate to increase the density of thepreform. The preform may be made, inter alia, by cold pressing, coldpressing and sintering, or hot pressing.

[0008] High pressure and high temperature processing of cubic perovskitepreforms has an advantage in sintering of the preforms to higherdensities. Another advantage is that high pressure may accomplish thesintering at lower temperatures, thus preserving stoichiometric andpurity. A further advantage is that sintering at higher temperatures canbe done without dissociation of the compound. Further sintering at highpressures will minimize grain growth so that fine-grained dense compactscan be produced. Fine grain size and high density are very important fortargets used in production of thin films with uniform thickness. Theseand other advantages will be readily apparent to those skilled in theart based upon the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0009] SrRuO₃ belongs to class of materials called cubic perovskites.The principle perovskite structure found in ferroelectric materials is asimple cubic structure containing three different ions of the form ABO₃.The A and B atoms represent +2 and +4 ions, respectively, while the Oatom is the O⁻² ion. This ABO₃ structure, in a general sense, can bethought of as face centered cubic (FCC) lattice with A atoms at thecorners and the O atoms on the faces. The B atom completes the pictureand is located at the center of the lattice. This A atom is the largestof the atoms and consequently increases the overall size of the AO₃(FCC) structure. As a result, there are minimum energy positions offcentered from the original octahedron that can be occupied by the Batom.

[0010] Different types of A and B elements include:

[0011] Dodecahedral A-site: Na⁺, K⁺, Rb⁺, Ag⁺, Ca⁺², Sr⁺², Ba⁺², Pb⁺²,La⁺³, Pr⁺³, Nb⁺³, Bi⁺³, Y⁺³, Ce⁺⁴, Th⁺⁴.

[0012] Octahedral B-site: Li⁺, Cu⁺², Mg⁺², Ti⁺³, V⁺³, Cr⁺³, Mn⁺³, Fe⁺³,Co⁺³, Al⁺³, Ni⁺³, Rh⁺³, Hf⁺⁴, Ti⁺⁴, Zr⁺⁴, Mn⁺⁴, Ru⁺⁴, Pt⁺⁴, Nb⁺⁵, Ta⁺⁵,Mo⁺⁶, W⁺⁶.

[0013] The atomic structure of ferroelectric thin films is verysensitive to fluctuations in the temperature of the crystal. As thetemperature is changed, the crystallographic dimensions change due todistortion of the octrahedra. These different crystal structures are:cubic, tetragonal, orthorhombic, and rhombohedral. As a result, thedistorted octrahedra are coupled together, and a very large spontaneouspolarization can be achieved. This large spontaneous polarization willlead to a large dielectric constant, with sensitive temperaturedependence.

[0014] Reported applications of perovskites include: BaTiO₃, multilayercapacitor; Pb(Zr, Yi)O₃, piezoelectric transducer; BaTiO₃; electro-opticmodulator, (Pb, La)(Zr, Ti)O₃; LiNbO₃, switch; BaZrO₃, dielectricresonator; BaRuO₃, thick film resistor; Pb(Mg, Nb)O₃, electrostrictiveactuator; Ba(Pb, Bi)O₃ layered cuprates, superconductor; GdFeO₃,magnetic bubble memory; YAlO₃, laser host; (Ca, La)MnO₃, refractoryelectrode; and KNbO₃, second harmonic generator. While most perovskiteshave high electrical resistivities that make them good as dielectrics,some are considered to be good conductors and semi-conductors. Some ofthe best perovskite conductors are the cubic sodium tungsten bronzes(Na_(x)WO₃; where x is between 0.3 and 0.95). In these bronzes, theresistivity tends to minimize at 0.7 mole-% of Na.

[0015] The perovskite material subjected to the HP densificationprocess, or feedstock, can be in powder form or can be converted into apreform for ease in handling. A “preform” is a coherent mass of materialin a pre-determined shape. The preform most often is made by pressing ofthe feed material, optionally, with heat added during the pressing orafterwards (sintering). Regardless of the technique of choice in formingthe preform, such preform (or the powder itself) next is placed within ashield or enclosure material (Ta, Grafoil) and the wrapped preformplaced within the reaction cell of the HP/HT apparatus.

[0016] The basic high pressure/high temperature (HP/HT) manufacturingmethod of the type herein involved entails the placing of a mass of feedcubic perovskite within a protectively shielded enclosure which isdisposed within the reaction cell of an HP/HT apparatus of a typedescribed further in U.S. Pat. Nos. 2,947,611; 2,941,241; 2,941,248;3,609,818; 3,767,371; 4,289,503; 4,673,414; and 4,954,139. The contentsof the cell then are subjected to processing conditions selected assufficient to affect a sintering and densification of the cubicperovskite. Such processing conditions generally involve the impositionof a pressure above about 2 Kbar and temperatures above about 800° C.for at least about 3 minutes. Useful pressures can be expected to rangefrom about 2 to about 75 Kbar with corresponding useful temperaturesfrom between about 800° to about 1600° C. Pressing times should rangefrom about 3 minutes to about 24 hours.

[0017] Following such high-pressure excursion, the temperature initiallyis lowered followed by release of the pressure. The shield material thencan be ground off or dissolved in a solvent therefor and the densifiedcubic perovskite recovered. The densified cubic perovskite then can bemachined or otherwise processed according to its intended use.

[0018] While the invention has been described with reference to apreferred embodiment, those skilled in the art will understand thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. In this application all units are in the metric system and allamounts and percentages are by weight, unless otherwise expresslyindicated. Also, all citations referred herein are expresslyincorporated herein by reference.

EXAMPLES Example 1

[0019] Sample #1 was in the form a cylindrical disc of SrRuO₃, which hada density of 3.16 gm/cc and the following dimensions: diameter=0.518inch and thickness=0.085 inch. The sample was encased in 0.004-inchthick molybdenum foil to prevent reaction with surrounding high-pressuremedium. Further, 0.015 inch thick Grafoil parting layer was placedaround the molybdenum foil. The encased disc was placed in the highpressure cell containing graphite as pressure transmitting medium andpressed at ˜60 kb and 120° C. for 25 minutes. The dimensions of thesample after pressing were: diameter=0.505 inch, thickness=0.050 inch,and the weight was 0.880 gm. This sample has a density of 5.36 gm/cc(theoretical density of 6.489 g/cc). Hence, the density of the samplewas increased from 3.16 g/cc to 5.36 g/cc (82.6% of theoretical density)by high pressure and high temperature processing.

Example 2

[0020] Sample # 6 was in the form a cylindrical disc of SrRuO₃. The discwas friable and no density on it could be determined. The disc wasencased in 0.004″ thick molybdenum foil and placed in a high-pressurecell containing salt (sodium chloride) as the pressure-transmittingmedium. The sample was subjected to ˜60 Kbar pressure and 120° C. for 35minutes.

[0021] The sample was recovered from the reaction cell and themolybdenum encasement was removed by grinding. The density of theproduct, as measured by immersion technique, was 6.36 gm/cc. X-raydiffraction analysis of the sample confirmed that it is SrRuO₃. Thetheoretical density of SrRuO3 is 6.489 gm/cc. Hence, the sample ofSrRuO3 prepared by high pressure and high temperature processing was 98%of the theoretical density. The product also was phase pure.

1. Method for increasing the density of a perovskite, which comprisesthe steps of: (a) placing a perovskite feedstock in a high-pressure cellof a high pressure/high temperature (HP/HT) apparatus; (b) subjectingsaid feedstock to pressures in excess of about 2 Kbar and temperaturesabove about 800° C. for time in excess of 3 minutes to produce an cubicperovskite product having a density which is greater than said preform;and (b) recovering said perovskite product.
 2. The method of claim 1,wherein said perovskite can be represented by the structure, ABO₃,where: A is one or more of Na⁺, K⁺, Rb⁺, Ag⁺, Ca⁺², Sr⁺², Ba⁺², Pb⁺²,La⁺³, Pr⁺³, Nb⁺³, Bi⁺³, Y⁺³, Ce⁺⁴, or Th⁺⁴; and B is one or more of Li⁺,Cu⁺², Mg⁺², Ti⁺³, V⁺³, Cr⁺³, Mn⁺³, Fe⁺³, Co⁺³, Al⁺³, Ni⁺³, Rh⁺³, Hf⁺⁴,Ti⁺⁴, Zr⁺⁴, Mn⁺⁴, Ru⁺⁴, Pt⁺⁴, Nb⁺⁵, Ta⁺⁵, Mo⁺⁶, or W⁺⁶.
 3. The method ofclaim 2, wherein said preform is SrRuO₃.
 4. The method of claim 1,wherein said perovskite feedstock is one or more of powder or a preform.5. The method of claim 1, wherein said perovskite product has a densityof greater than about 60% of its theoretical density.
 6. The method ofclaim 5, wherein said perovskite product has a density of greater thanabout 90% of its theoretical density.
 7. The method of claim 1, whereinstep (b) is conducted for a time ranging from between about 3 minutesand 24 hours.
 8. The method of claim 1, wherein said pressure rangesfrom about 2 to 75 Kbar and said temperature ranges from about 800° to1600° C.
 9. The method of claim 7, wherein said pressure ranges fromabout 2 to 75 Kbar and said temperature ranges from about 800° to 1600°C.
 10. The densified perovskite product produced according to theprocess of claim
 1. 11. The densified perovskite product producedaccording to the process of claim
 2. 12. The densified perovskiteproduct produced according to the process of claim
 3. 13. The densifiedperovskite product produced according to the process of claim
 4. 14. Thedensified perovskite product produced according to the process of claim5.
 15. The densified perovskite product produced according to theprocess of claim
 6. 16. The densified perovskite product producedaccording to the process of claim
 7. 17. The densified perovskiteproduct produced according to the process of claim
 8. 18. The densifiedperovskite product produced according to the process of claim
 9. 19.Method for increasing the density of a perovskite, which comprises thesteps of: (a) placing a perovskite feedstock in a high-pressure cell ofa high pressure/high temperature (HP/HT) apparatus; (b) subjecting saidfeedstock to pressures in excess of about 2 Kbar and temperatures aboveabout 800° C. for time adequate to increase the density of saidfeedstock to above about 60% of its theoretical density; and (b)recovering said perovskite product having a density above about 60% ofit theoretical density.
 20. The method of claim 19, wherein saidperovskite can be represented by the structure, ABO₃, where: A is one ormore elements of Na⁺, K⁺, Rb⁺, Ag⁺, Ca⁺², Sr⁺², Ba⁺², Pb⁺², La⁺³, Pr⁺³,Nb⁺³, Bi⁺³, Y⁺³, Ce⁺ ⁴, or Th⁺⁴; and B is one or more elements of Li⁺,Cu⁺², Mg⁺², Ti⁺³, V⁺³, Cr⁺³, Mn⁺³, Fe⁺³, Co⁺³, Al⁺³, Ni⁺³, Ni⁺³, Rh⁺³,Hf⁺⁴, Ti⁺⁴, Zr⁺⁴, Mn⁺⁴, Ru⁺⁴, Pt⁺⁴, Nb⁺⁵ Ta⁺⁵, Mo⁺⁶, or W⁺⁶.
 21. Themethod of claim 19, wherein said preform is SrRuO₃.
 22. The method ofclaim 19, wherein said perovskite feedstock is one or more of powder ora preform.
 23. The method of claim 19, wherein said perovskite producthas a density of greater than about 90% of its theoretical density. 24.The method of claim 19, wherein step (b) is conducted for a time rangingfrom between about 3 minutes and 24 hours.
 25. The method of claim 19,wherein said pressure ranges from about 2 to 75 Kbar and saidtemperature ranges from about 800′ to 1600° C.
 26. The method of claim25, wherein said pressure ranges from about 2 to 75 Kbar and saidtemperature ranges from about 800° to 1600° C.